1. Field of the Invention
This invention relates generally to start-up circuits and, more specifically, the present invention relates to start-up circuits for reference circuits having internal nodes that need to be charged to a predetermined voltage level by incorporating a current driven design.
2. Description of the Prior Art
In certain types of electrical circuits, there are internal nodes that need to be quickly charged to a predetermined voltage level. The normal application of power to a circuit, typically VDD, results in a slow ramping of voltage that is insufficient to effectuate the proper operation of the reference circuit.
The prior art describes circuits with simple diode feedback loops with low gain circuits that do not satisfactorily solve the problem of slow voltage ramp up. Furthermore, these start-up circuits are characterized as voltage driven circuits and contain no overdrive capability.
The invention described in U.S. Pat. No. 5,742,155 entitled "ZERO CURRENT START-UP CIRCUIT" also describes a voltage driven circuit. However, unlike the prior art, this invention discloses overdrive capability that initially drives the output voltage of the high gain amplifier to a much higher level than required. The feedback loop returns the voltage level to that needed by the reference circuit.
The reference circuit for the previously patented invention was intended to drive a voltage reference circuit such as a bandgap or other types. This application required a current mirror with a diode and resistor pair in series to regulate the output voltage to 1.2 volts. The present invention, not specifically intended for a voltage reference, does not incorporate this circuitry.
The previously patented invention uses a voltage feedback implementation and is therefore voltage driven. The present invention relies on a current feedback approach and is therefore current driven. This approach results in a start-up circuit with potentially wider application.
The previously patented invention also relies on logic gate means to effectuate the feedback mechanism. In particular embodiments, the logic gate means can include a latch device, which could prohibit start-up if the latch was initialized to an erroneous state. The present invention does not rely on latches or logic gate means for the feedback loop.
Therefore, a need existed to provide a fast start-up circuit that incorporates a new approach and new circuitry not found in the prior art.